Fixes #14184: Module service locator now falls back to its parent module service locator in case component isn't found

docs/guide/concept-service-locator.md

@@ -12,7 +12,7 @@ The most commonly used service locator in Yii is the *application* object, which
 `urlManager` components. You may configure these components, or even replace them with your own implementations, easily
 through functionality provided by the service locator.
 
-Besides the application object, each module object is also a service locator.
+Besides the application object, each module object is also a service locator. Modules implement [tree traversal](#tree-traversal).
 
 To use a service locator, the first step is to register components with it. A component can be registered
 via [[yii\di\ServiceLocator::set()]]. The following code shows different ways of registering components:
@@ -118,3 +118,13 @@ return [
 This alternative approach is most preferable when you are releasing a Yii component which encapsulates some non-Yii
 3rd-party library. You use the static method like shown above to represent the complex logic of building the
 3rd-party object, and the user of your component only needs to call the static method to configure the component.
+
+## Tree traversal <span id="tree-traversal"></span>
+
+Modules allow arbitrary nesting; a Yii application is essentially a tree of modules.
+Since each of these modules is a service locator it makes sense for children to have access to their parent.
+This allows modules to use `$this->get('db')` instead of referencing the root service locator `Yii::$app->get('db')`.
+Added benefit is the option for a developer to override configuration in a module.
+
+Any request for a service to be retrieved from a Module will be passed on to its parent in case the module is not able to satisfy it.
+ 

docs/guide/structure-modules.md

@@ -269,6 +269,34 @@ in the `admin` module which is a child module of the `forum` module.
 to its parent. The [[yii\base\Application::loadedModules]] property keeps a list of loaded modules, including both
 direct children and nested ones, indexed by their class names.
 
+## Accessing components from within modules
+
+Since version 2.0.13 modules support [tree traversal](concept-service-locator.md#tree-traversal). This allows module 
+developers to reference (application) components via the service locator that is their module.
+This means that it is preferable to use `$module->get('db')` over `Yii::$app->get('db')`.
+The user of a module is able to specify a specific component to be used for the module in case a different component
+(configuration) is required.
+
+For example consider this application configuration:
+
+```php
+'components' => [
+    'db' => [
+        'tablePrefix' => 'main_',
+    ],
+],
+'modules' => [
+    'mymodule' => [
+        'components' => [
+            'db' => [
+                'tablePrefix' => 'module_',
+            ],
+        ],
+    ],
+],
+```
+
+The application database tables will be prefixed with `main_`, while all module tables will be prefixed with `module_`.
 
 ## Best Practices <span id="best-practices"></span>
 

framework/CHANGELOG.md

@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.13 under development
 ------------------------
 
+- Enh #14184: Module service locator now falls back to its parent module service locator in case component isn't found (SamMousa)
 - Bug #14596: Fix event call on init in `yii\widgets\BaseListView` (panchenkodv)
 - New #14151: Added `AttributesBehavior` that assigns values specified to one or multiple attributes of an AR object when certain events happen (bscheshirwork)
 - Bug #6526: Fixed `yii\db\Command::batchInsert()` casting of double values correctly independent of the locale (cebe, leammas)

framework/base/Module.php

@@ -711,4 +711,22 @@ class Module extends ServiceLocator
         $this->trigger(self::EVENT_AFTER_ACTION, $event);
         return $event->result;
     }
+
+    /**
+     * @inheritdoc
+     * Since version 2.0.13, if a component isn't defined in the module, it will be looked up in the parent module.
+     * The parent module may be the application.
+     */
+    public function get($id, $throwException = true)
+    {
+        if (!isset($this->module)) {
+            return parent::get($id, $throwException);
+        }
+        
+        $component = parent::get($id, false);
+        if ($component === null) {
+            $component = $this->module->get($id, $throwException);
+        }
+        return $component;
+    }
 }

framework/di/ServiceLocator.php

@@ -40,6 +40,7 @@ use yii\base\InvalidConfigException;
  * ```
  *
  * Because [[\yii\base\Module]] extends from ServiceLocator, modules and the application are all service locators.
+ * Modules add [tree traversal](guide:concept-service-locator#tree-traversal) for service resolution.
  *
  * For more details and usage information on ServiceLocator, see the [guide article on service locators](guide:concept-service-locator).
  *

tests/framework/base/ModuleTest.php

@@ -9,6 +9,8 @@ namespace yiiunit\framework\base;
 
 use Yii;
 use yii\base\Controller;
+use yii\base\Module;
+use yii\base\Object;
 use yiiunit\TestCase;
 
 /**
@@ -84,6 +86,17 @@ class ModuleTest extends TestCase
         $this->assertNotNull(Yii::$app->controller->action);
         $this->assertEquals('test/test-controller1/test1', Yii::$app->controller->action->uniqueId);
     }
+
+
+    public function testServiceLocatorTraversal()
+    {
+        $parent = new Module('parent');
+        $child = new Module('child', $parent);
+        $grandchild = new Module('grandchild', $child);
+
+        $parent->set('test', new Object());
+        $this->assertInstanceOf(Object::className(), $grandchild->get('test'));
+    }
 }
 
 class TestModule extends \yii\base\Module